Move threatens a fragile ceasefire between Russia and Turkey to forestall a Syrian government offensive

BEIRUT—A powerful extremist group seized control of most of Syria’s last opposition stronghold on Thursday, threatening a fragile cease fire intended to avert a Syrian military offensive and likely humanitarian catastrophe.

Russia and Turkey reached a deal in September to create a demilitarized buffer zone in northwest Idlib province to forestall a Syrian government offensive against rebel groups, many of whom are backed by Ankara. The truce has largely held since mid-October despite the activities of one terrorist group there—Hayat...